1. Field of the Invention
The present invention relates to optical waveguides, an optical module, and a method for producing the optical module. More particularly, the present invention relates to: optical waveguides for causing an optical signal for use in optical communication or the like to branch off or coupling such optical signals when such optical signals propagate therethrough; an optical module using such an optical waveguide; and a method for producing such an optical module.
2. Description of the Background Art
Recent years have seen optical communication systems come into wide use in public communication, a computer network, etc. In order to realize high-speed performance and high functionality, such optical communication systems utilize broadband optical communication and have a wavelength division multiplex optical transmission function and/or an interactive optical transmission function. In the field of optical communication, studies of optical integrated circuits having various functions have been eagerly conducted for performing advanced signal processing. An optical integrated circuit includes an optical waveguide as a basic element. In the optical waveguide, a core region having a high refractive index is covered with a cladding layer having a relatively low refractive index, and light is confined within the core region and caused to propagate therethrough. Various functions are realized by arranging cores in a patterned manner. Among others, a quartz-based optical waveguide is typical of passive optical waveguides having various advantages, such as low loss properties, physical and chemical stability, and conformability to an optical fiber. In a typical method for producing an optical waveguide, flame deposition is used for forming a core cladding film and reactive ion etching is used for forming a core pattern. In addition to the flame deposition, suggested methods for forming the core cladding film include CVD (chemical vapor deposition), vacuum deposition, sputtering, etc.
Technologies for realizing a small, multifunctional, and low-cost optical waveguide by mounting an optical semiconductor element, etc., on the optical waveguide are considered as being promising and are currently under eager study. For example, Japanese Patent Laid-Open Publication No. 11-68705 discloses a module aimed at integration by mounting optical semiconductor elements, such as a semiconductor laser and a photodiode, on an Si substrate or an optical waveguide platform which is formed on the SI substrate and has the quartz-based optical waveguide formed therein.
FIG. 10 is a view illustrating a structure of the optical module disclosed in Japanese Patent Laid-Open Publication No. 11-68705. In the optical module illustrated in FIG. 10, an optical waveguide substrate 102 including a quartz-based optical waveguide core 102c is formed using flame deposition or the like, such that the optical waveguide substrate 102 and an Si substrate 101, which is previously processed by etching or the like, are stacked. A laser diode (LD) 103 and a photodiode (PD) 104 are mounted in spaces provided by removing surplus portions of a quartz-based material deposited as the optical waveguide substrate 102. The LD 103 and the PD 104 are mounted on the Si substrate 101 or the optical waveguide substrate 102.
The optical waveguide core 102c has an end surface 102ca formed in a side face of the optical waveguide substrate 102. The optical waveguide core 102c also has an end surface 102cb formed in another side face of the optical waveguide substrate 102 which is formed in the space where the PD 104 is mounted. The optical waveguide core 102c has a branch which is formed such that an optical signal propagating therethrough is reflected by a reflecting plane 102cr, which is formed at an intermediate point of the optical waveguide core 102c formed between the end surf aces 102ca and 102cb, so as to be transmitted to the end surface 102ca. The branch of the optical waveguide core 102c connects the reflecting plane 102cr with still another side face of the optical waveguide substrate 102, which is formed in the space where the LD 103 is mounted, and forms an end surface 102cc in the same side face.
The reflecting plane 102cr formed in a branching portion of the optical waveguide core 102c includes a portion of an optical filter 105. A groove is formed by processing, such as dicing, in the stacked substrates 101 and 102. The optical filter 105 is inserted into the groove. The optical filter 105 is then fixed in the groove. Specifically, the optical waveguide core 102c formed between the end surfaces 102ca and 102cb is disconnected at the intermediate point by the groove which is filled by inserting the optical filter 105. Typically, the optical filter 105 has an area sufficiently larger than a cross section of the optical waveguide core 102c. The reflecting plane 102cr is formed by bonding a portion of the optical filter 105 to the cross section of the optical waveguide core 102c at opposite sides of the optical filter 105. In order for the optical module to have a wavelength division multiplex function, etc., the optical filter 105 has properties of transmitting therethrough an optical signal, which has a wavelength within a prescribed wavelength range, and reflecting an optical signal having a wavelength out of the prescribed wavelength range.
Propagation of an optical signal through the optical module will now be described. The LD 103 outputs toward the end surface 102cc an optical signal having a wavelength within a range of wavelengths to be reflected by the optical filter 105. The optical signal output by the LD 103 propagates from the end surface 102cc through the branch of the optical waveguide core 102c and is incident on the reflecting plane 102cr. The wavelength of the optical signal lies within the range of wavelengths to be reflected by the optical filter 105, and therefore the optical signal incident on the reflecting plane 102cr is reflected by the reflecting plane 102cr, so as to propagate through the optical waveguide core 102c towards the end surface 102ca, and exits therefrom.
On the other hand, in the case where an optical signal, which has a wavelength out of the range of wavelengths to be reflected by the optical filter 105, enters the optical waveguide core 102c from the end surface 102ca, the optical signal propagates from the end surface 102ca through the optical waveguide core 102c so as to be incident on the reflecting plane 102cr. The wavelength of the optical signal lies out of the range of wavelengths to be reflected by the optical filter 105, and therefore the optical signal incident on the reflecting plane 102cr is transmitted therethrough, so as to propagate through the optical waveguide core 102c toward the end surface 102cb, and exits therefrom. The optical signal having exited from the end surface 102cb is incident on the PD 104.
However, the branching portion of the optical waveguide core 102c of the optical module is required to have a fine and highly accurate pattern formed therein in order to suppress the loss of an optical signal propagating therethrough. For example, regarding an optical signal, which is output by the LD 103 and reflected by the reflecting plane 102cr so as to exit the optical module from the end surface 102ca, an angle between the optical waveguide core 102c, which is formed between the end surfaces 102ca and 102cb, and the branch thereof is required to coincide with a reflection angle of the optical signal. The reflection angle is significantly influenced by angles between the optical filter 105, the optical waveguide core 102c, and the branch. The angles of the optical filter 105 with respect to the optical waveguide 102c and the branch are determined by the groove formed by processing the Si substrate 101 and the optical waveguide substrate 102 which are stacked as described above. The groove is required to be formed by highly accurate processing. The optical filter 105 is preferably inserted into the groove without making any gaps between them. However, it is very difficult to realize such stable dimensional accuracy, and therefore the insertion of the optical filter 105 into the groove often leaves slight gaps between them. In such a case, the gaps between the optical filter 105 and the groove are filled with an adhesive or the like which does not influence the propagation of the optical signal. However, even when the gaps are filled with the adhesive or the like, the angles of the optical filter 105 with respect to the optical waveguide core 102c and the branch thereof greatly vary depending on warping or inclination of the optical filter 105 within the groove.
In the case where the optical signal is on a single mode, it is necessary to perform alignment, assembly, and fixation with an accuracy of ±1 micrometer (μm) or less in order to suppress the loss of the optical signal propagating through the above-described optical module. In order to satisfy requirements for such alignment accuracy, a production machine, such as a highly accurate processor, is currently required. Positional adjustments, such as alignment of optical axes, are performed by a system including a multiple-axis automatic alignment mechanism, and there are significant problems in aspects of mass-productivity and economical efficiency of the optical module. Accordingly, the optical module is not suitable for mass production and it is difficult to realize a low-cost optical module.